**5. Conclusions**

The addition of increased concentrations of NaCl to OMW/glycerol blends, a novel application, showed an interesting physiological response of the yeas<sup>t</sup> *Y. lipolytica*. In essence, the combined effect of the addition of OMWs and NaCl into the medium increased lipid in DCW values of *Y. lipolytica* ACA-YC 5031, while remarkable biomass production was observed in all trials. Although glycerol metabolism, in the experiment with no OMWs and no salt added, was mainly shifted toward the synthesis of polyols, the addition of OMWs resulted in a reduction of <sup>Σ</sup>polyols concentration and a significant increase in the production of citric acid. The addition of salt in the OMW-based media resulted in a drastic increase in citric acid production. reaching maximum values in the trial with the addition of 5.0% NaCl. These results indicate that the yeas<sup>t</sup> *Y. lipolytica* is a grea<sup>t</sup> candidate for the production of value-added compounds. The results showed that the addition of NaCl positively affected the physiological behavior of the yeast, having a grea<sup>t</sup> impact on the production of citric acid and lipids by this microorganism. Additionally, OMWs were found to be a promising substrate for the production of citric acid, mainly combined with salt. Finally, simultaneous with the production of several added-value metabolites, the non-negligible color and phenolic compounds removal from the medium with the aid of *Y. lipolytica* suggested that the yeas<sup>t</sup> can also be used as an important microbial cell factory amenable for the depollution and detoxification of several phenol-rich recalcitrant wastewaters that are frequently generated during various agro-industrial activities.

**Author Contributions:** Conceptualization, S.P.; Methodology, M.K., S.P. and D.S; Validation, M.K and S.P.; Formal Analysis, M.T, M.K. and S.P.; Investigation, M.K.; Resources, A.A.K. and S.P.; Writing – Original Draft Preparation, M.T. and S.P.; Writing – Review & Editing, M.T. and S.P.; Supervision, D.S., A.A.K. and S.P.; Project Administration, A.A.K. and S.P; Funding acquisition, S.P.

**Funding:** This research was funded by the project entitled "Adding value to biodiesel-derived crude glycerol with the use of Chemical and Microbial Technology" (Acronym: Addvalue2glycerol; gran<sup>t</sup> number T1EΔK-03002), which is financially supported by the Ministry of National Education and Religious Affairs, Greece (project action: "Investigate – Create – Innovate 2014-2020, Intervention II").

**Conflicts of Interest:** The authors declare no conflict of interest.
